This invention relates to a non-volatile semiconductor memory device having a double-layer gate structure which includes a gate insulating film, a floating gate layer as a charge storing layer, an insulating film, and a control gate layer. It also relates to a method for manufacturing the device. More particularly, the invention relates to a structure which includes gate insulating films and gate electrodes incorporated in a memory cell section and its peripheral circuit section.
Non-volatile semiconductor memory devices each comprise a memory cell (a memory cell transistor), a select transistor, and a peripheral circuit including a transistor of a high breakdown voltage (Vpp) and a transistor operable with a normal power Vcc (the transistors of the peripheral circuit will hereinafter be referred to as “peripheral circuit transistors”). These transistors have gate insulating films of different thicknesses corresponding to voltages applied thereto.
FIGS. 36A, 36B and FIGS. 37A and 37B are sectional views, illustrating the conventional steps of manufacturing a nonvolatile semiconductor memory device. As is shown in FIG. 36A, N-well regions 302 and P-well regions 303 are formed in a silicon substrate 301, and then sufficiently thick element isolating films 304 are formed by the LOCOS method. Element regions isolated by the element isolating films 304 include, for example, a memory cell section, a select transistor (select Tr), and transistors incorporated in a memory peripheral circuit, such as a high breakdown voltage transistor (Vpp Tr) and a normal power transistor (Vcc Tr). First, a gate oxide film 305 is formed for the select Tr. Then, resist is coated and patterned, thereby covering the region other than a memory cell section with a resist layer 315, removing the gate oxide film 305 and forming a gate oxide film 306 for the memory cell section. In FIGS. 36 and 37, each gap indicates that the memory cell and the select Tr, Vpp Tr and Vcc Tr show different sections.
Referring then to FIG. 36B, a polysilicon layer 307 as a first layer is deposited on the resultant structure and then patterned. Thereafter, an insulating film 308 is formed on each of the patterned polysilicon layers. The polysilicon layers 307 serve as the floating gate of the memory cell and the gate electrode of the select transistor. On the transistor (Vpp Tr, Vcc Tr) side of the peripheral circuit, the insulating film 308, the polysilicon layer 307 as the first layer, and the gate insulating film 305 below the layer 307 are removed. Thereafter, the resist is patterned, thereby forming a gate oxide film 309 in the Vpp Tr section. Further, another resist layer 316 is patterned as shown in FIG. 36B, thereby removing the gate oxide film 309 in the Vcc Tr section.
Subsequently, as shown in FIG. 37C, a gate oxide film 310 is formed in the Vcc Tr section, and then a polysilicon layer (gate electrode) 311 as a second layer is formed. Thereafter, the memory cell section and each transistor section are patterned, ion implantation is performed, an interlayer insulating film 312 is deposited, and wiring layers 313 are formed. Thus, a memory cell, a select transistor, a high breakdown voltage transistor and a Vcc transistor are formed as shown in FIG. 37D.
In the above-described structure, the four gate oxide films 305, 306, 309 and 310 of the transistors are formed in different steps. Therefore, a great number of resist forming steps, oxidation steps, etc. are required, resulting in an increase in manufacturing cost.
Moreover, as described above, in the non-volatile semiconductor memory device having a memory cell section of a double-layer gate structure consisting of a floating gate layer (first polysilicon layer 307) and a control gate layer (second polysilicon layer 311), the gate electrodes of the transistors in the peripheral circuit are usually realized by the use of the control gate layer (second polysilicon layer 311) in the memory cell section. If in this case, surface-channel type N-channel and P-channel MOS transistors are formed as the transistors of the peripheral circuit, the following difficulties will occur:
In general, the control gate layer of a memory cell transistor has a polycide structure formed by depositing, for example, WSi (tungsten silicide) on the second polysilicon layer to increase the conductivity. Then, the control gate layer is coated with resist and then patterned into a gate electrode.
In the conventional method using the control gate layer as the gates of the transistors of the peripheral circuit, it is necessary before the deposition of WSi to correctly implant each of N-type and P-type impurities into the second polysilicon layer, in order to form N-channel and P-channel MOS transistors of surface-channel type suitable for integration. Then, it is necessary to deposit WSi, and to correctly implant each of N-type and P-type impurities into regions which will serve as source and drain regions, after the gate electrode is formed. Thus, the step of patterning resist and the step of implanting impurities must be repeated.
If, on the other hand, the gate electrodes of the transistors of the peripheral circuit are formed of the first polysilicon layer 307 which will serve as the floating gate layer of the memory cell, a surface-channel type element can be obtained by implanting, into the gate electrode, an impurity of the same conductivity as that of an impurity implanted in the source and drain regions, after the gate electrode is formed. In this case, however, the high speed operation of the transistors of the peripheral circuit cannot be realized, since the first polysilicon layer 307 as the floating gate layer usually has a higher resistance than the second polysilicon layer 311 as the control gate layer.
As explained above, in the conventional method, the transistors of the peripheral circuits have gate insulating films of different thicknesses, which inevitably increases the manufacturing steps and hence the manufacturing cost. To achieve high speed operation, the gate of each transistor of the peripheral circuit of the memory usually has the same polycide structure as the control gate layer of the memory cell section. If a surface-channel type element is realized by the transistor with the gate of the polycide structure, a great number of resist patterning steps and impurity implanting steps are required, thereby increasing the manufacturing cost.